In the prior art, various systems and methods have been devised to control automatic or driverless vehicles. One system uses in-floor wire guidance to force an automatic guided vehicle (AGV) to follow one or more wires in the floor. This is accomplished by having each wire carry a small electric current of a different, but known, frequency. The wire(s), in turn, are installed and sealed in narrow, shallow slots cut in the floor. The cut slots, made up of arcs and straight line segments, are defined by the requirements of the system's desired guidepath(s). Typically, the various arcs and straight line segments are made tangential to each other in order to minimize the steering effort required to traverse a given path.
Tunable inductive sensors are then used to control the steering of each vehicle such that it will follow the radiated electromagnetic field of the desired wire (frequency). By using various frequencies to describe different sections of the guidepaths, the vehicle's route can be controlled by telling it which frequency to follow.
In order to know when to switch frequencies, the vehicle must also know where it is along the desired wire guidepath. Since the in-floor wire only provides the vehicle with lateral (steering) position information, position markers are used to update the vehicle's position along the wire guidepath. These position markers may be electromagnetic transponders, magnets, optic reflectors, etc. In any case, when a vehicle passes such a marker, it knows its exact location along the wire guidepath. These locations, in turn, can be used to enable or command the vehicle to switch frequencies, change speed/direction, stop, pickup, drop off, and/or perform one or more of many other possible commands. Typically, the particular operation(s) to be performed at each specific location, either immediately or delayed, would be described by an entry in an internal vehicle program (VP) database which, in turn, would be addressed, directly or indirectly, by the position marker itself.
Another system that employs AGVs is a non-wire control system. In these systems, instead of a wire placed in the floor, the AGV is directed along a virtual (logical) guidepath using reference markers for absolute position corrections and a vehicle controller that governs the movement of the AGV based on one or more inputs to the controller. These non-wire systems offer significant advantages over wire systems since no costs are incurred to lay wire in a floor.
U.S. Pat. Nos. 5,281,901 and 5,341,130 to Yardley et al. teach an automated guided vehicle control system which is downward compatible with existing guidewire systems providing both guidewire navigation and guidance and automatic navigation and guidance and wireless communication between a central controller and each vehicle. The non-wire guidance mode of Yardley, et al., comprises a series of reference markers positioned in a floor, a number of non-wire AGVs and an AGV central controller. The controller keeps track of the status and position of each AGV in the system using a map of the guidepath layout which is stored in its memory. The central controller provides two-way wireless communication to each AGV in the system. Each AGV is equipped with a non-wire navigation and guidance system for controlling the AGV's movement along a selected guidepath. Reference markers are positioned in the floor at various points along the guidepath. These reference markers provide absolute positions for correcting the AGV's current position.
Each AGV has an on-board controller which calculates a guidepath based on: the vehicle's current X,Y-coordinate and heading; and the exit X,Y-coordinate and exit bearing angle of the next desired guidepath segment. This path segment exit point positioning and exit bearing information is transmitted to the AGV from the central controller. The on-board computer first selects the type of guidepath calculation to perform based on the AGV's current heading angle with respect to the designated bearing at the exit of the path segment. Once the type of guidepath calculation is selected, the on-board computer calculates a guidepath based on the transmitted exit point positioning information and the AGV's current position, as determined from shaft encoders, the gyro, and reference markers.
Since AGV's do not usually follow the planned guidepath perfectly between path points or destinations, a new unique guidepath is calculated each time an AGV enters a new guidepath segment with different position and/or error values. The calculation parameters for calculating a guidepath for a path segment are that the initial angle of the guidepath must be tangent to the current direction of the AGV and the exit angle of the guidepath must be tangent to the exit bearing supplied by the central controller. These patents use a fifth order polynomial equation to calculate the guidepoints of the guidepath along which the AGV steers.
Upon receipt of a load movement task from a management computer, the central controller selects an AGV and schedules an optimum path for the selected AGV. Based upon the path scheduled and the current position of the AGV, the central controller provides path segment by path segment control using the calculated guidepoints through two-way communications between the central controller and the AGV. The length of each path segment ranges from a fraction of the length of an AGV to a length greater than the AGV length.
Control Engineering Company, a subsidiary of Jervis B. Webb Company of Farmington, Michigan, also has a non-wire AGV control system comprising a system computer, a series of reference markers positioned in a floor and a number of non-wire AGVs. The system computer serves as a dispatcher and traffic controller for the system and provides two-way wireless communication to each AGV in the system. The AGV's are equipped with a non-wire navigation and guidance system for controlling the AGVs along the defined guidepaths in the system. The reference markers are positioned in the floor at various points along the guidepaths and provide absolute position references for the AGVs.
Although the Control Engineering Company system is similar to traditional or wire-guided systems, the error correction routine in its guidance system differs in that the error correction is compared to guidepath information supplied by a computerized model of the guidepaths on-board the AGV rather than data returned from sensing a guidewire.
Each AGV's on-board controller contains a database that is used to simulate functions previously achieved by sensing the guidewire and position markers in a floor. The database includes details of the position of the location markers in the floor and absolute X,Y coordinates of the termination points for every path segment in the system. The database also includes function and location, in terms of distance traveled with respect to a path point, for each point in the system at which an AGV operation is to be performed. Typical functions include stopping, operating an on-board conveyor, resetting a release command, or the like.
The database is developed on a computer-aided design system with the guidepaths being designed so that each path segment in the system terminates tangentially with respect to an adjacent path segment. This restriction on the guidepath layout permits each path segment in the guidepath system to be characterized by absolute X,Y coordinates of its termination points and basic geometry. The X,Y coordinates for all the path segments in the system are stored in the database in each of the AGVs in the system.
In operation, the system computer transmits a final destination command and a route-release command to the AGV. The final destination command includes a vehicle identifier, a destination number, and an operation. The vehicle identifier contains the address of the specific vehicle the command is being issued to. The destination number is a number associated with a location in the system which may identify a conveyor, a queue, or a stop location. An operation designates an action to be performed by the vehicle such as a horn beeping or operation of an on-board conveyor or the like. The route release command is used to control the vehicle through path segments in the system. The route identifies a string of path-point labels which will define to the vehicle a portion of its desired path. When a route release is granted, the associated path points will extend the vehicle's current path toward its final destination or next hold stop position. As the AGV approaches the end of a route, the system controller will transmit another route-release command for the AGV. The AGV will acknowledge receipt of the command and proceed to negotiate this route. This process will continue until the AGV reaches its final destination which was transmitted as part of the destination command.
As described above, the route may comprise several path segments. The on-board computer of the AGV utilizes the absolute X,Y-coordinates retrieved from the database to regenerate the previously designed guidepath for each path segment. The AGV's guidance system will routinely check and correct the movement of the AGV so that it proceeds along the guidepath. The guidepaths for all path segments in the system are defined before the system is placed into operation. consequently, there is no selection of guidepaths made by the AGV's navigation and guidance system or a calculation of a guidepath based on an exit bearing from a path segment.
The non-wire systems described above are not without their disadvantages. The system described in the Yardley, et al., patents utilizes a complex 5th degree polynomial for describing a non-wire guidepath. In addition, since the Yardley, et al., system, when receiving exit-point positioning information, proceeds to select and calculate a guidepath based on the exit point's X,Y coordinates and exit bearing and the vehicle's actual position, the vehicle could calculate a "new path" into an obstacle provided its current position and/or orientation errors are great enough. Further, since this system uses not only X,Y coordinates but also exit bearings, much information must be transmitted from the central controller to the AGVs in order to direct their movement along a non-wire path. This information load can limit the number of vehicles controlled by a controller and may cause reliability problems and/or broadcast difficulties in the system.
Non-wire systems which have on-board controllers using absolute X,Y coordinates suffer from the need of requiring vast amounts of memory to accommodate the information required for the database. Again, since large amounts of information are required, transmitting this information from a remotely located controller causes the same problems as described above with the Yardley, et al., system.
In view of the drawbacks in the prior art non-wire systems, a need has developed to provide an improved system and method for controlling automatic guided vehicles. The present invention overcomes the disadvantages in the prior art noted above by providing a system and method for describing, generating, and checking non-wire guidepaths for automatic guided vehicles which simulates the simple arc and straight line features of an in-floor wired guidepath while also being compact enough to be held in a relatively small database either on the vehicle or on a common system controller remotely located from the vehicle.